Mobile irrigation systems having elevated boom or truss assemblies carrying multiple sprinklers are typically of the center pivot-type or the linear- (or lateral-) move-type. In a center-pivot machine, the elevated truss assembly pivots about an upright standpipe that supplies water to the sprinklers attached to the truss assembly. In a linear-move machine, the elevated truss assembly is carried on mobile, wheeled towers that move the machine linearly along a path that is perpendicular to the elevated boom or truss. Typically, the linear-move machine travels from one end of a field to the other and back again, and sprinkling typically occurs in both directions, with water supplied by hose, spaced hydrants or adjacent water-filled ditch or culvert.
In still other systems, a mobile traveler or cart moves along an above-ground pipe, docking with hydrants spaced along the pipe. See, for example, commonly-owned U.S. Pat. Nos. 7,300,004 and 7,140,563 incorporated herein by reference. In a variation of this arrangement, the pipe is below ground, eliminating the primary steering component used by the cart to move between hydrants. In some cases, buried guide wires are used to steer the cart. There remains a need, however, for a relatively simple and reliable system for guiding a mobile sprinkler cart along an above-ground pipe or across a field to engage a plurality of hydrants (in sequence) in accordance with a programmed sprinkling policy that controls the application of water to the field at each hydrant.